Closing unit for injection moulding machine

ABSTRACT

A closing unit for an injection moulding machine has a movable closing die and a fixed matching die, as well as several hydraulic cylinders mounted on the fixed matching die to generate the closing force. The closing die has a drawbar for each hydraulic cylinder. The piston of each hydraulic cylinder is mechanically linked to a rotary locking bushing. The drawbars extend through hydraulic cylinders and locking bushings. A servo-drive allows the locking bushings to swivel up to a first and second angular position. First locking elements located along a section of the drawbars cooperate with second locking elements in the locking bushings so that in the first angular position the drawbars may be axially pushed through the hydraulic cylinders and the locking bushings, and in the second angular position the closing force may be transmitted from the hydraulic cylinders to the closing die.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a closing unit for an injection molding machine.

2. Related Art

The closing unit of an injection molding machine receives the injection mold. It carries out the movements necessary for the closing and opening of the injection mold and produces the forces necessary for the locking and opening of the injection mold. The main components of each closing unit are a stationary plate on the injection side (hereinafter referred to as the injection plate), a movable closure plate, as well as a locking device. One part of the injection mold is clamped on the stationary injection plate while the complementary part of the injection mold is clamped on the movable closure plate. By locking device there is to be understood the device which, upon the injection and further pressing, produces the necessary closing force for keeping the injection mold closed.

Both mechanical locking devices with lever mechanisms and hydraulic locking devices with hydraulic cylinders are known.

The present invention relates to a closing unit with hydraulic locking. In the book “Kunstsfoff-Maschinen-Führer,” 3rd edition, edited by Dr. Eng. Friedrich Johannabe, published by Carl Hanser Verlag (Munich, Vienna), 1992 various constructions of closing units with hydraulic locking are described.

In most closing units with hydraulic locking, a force cylinder is developed as a pressure cushion and is arranged on the support plate behind the movable closure plate.

From FIG. 56, page 110, of said book, a closing unit having four hydraulic cylinders on the stationary injection plate is, however, known. The pistons of said hydraulic cylinders are connected rigidly via connecting rods with the movable closure plate. The four hydraulic cylinders produce the required closing force but also, at the same time, carry out the opening and closing movements of the movable closure plate. Since the four hydraulic cylinders at the same time perform a locking function and a displacement function, they must be both of large cross section and have a large stroke and they therefore take up a relatively large amount of space. The four relatively large hydraulic cylinders accordingly substantially impede accessibility to the injection unit and furthermore have a very high consumption of oil. The structural length of the closing unit is substantially greater than the greatest possible distance between the injection plate and the closure plate.

From FIG. 53B, page 108, of the same book, a more compact closing unit, also having four hydraulic cylinders on the stationary injection plate, is known. The four connecting rods are detachably connected by claws at their ends to the corresponding hydraulic cylinder. Opening and closing movements are produced, with connecting rods uncoupled, by high-speed cylinder arranged on the side. The stroke of the four hydraulic cylinders in this construction must accordingly correspond merely to the difference in length between the largest and smallest injection molds. This construction is primarily of interest when the injection molds used all have more or less the same length. If the closing unit, however, is to be used with injection molds of different length, the stroke of the hydraulic cylinders must be relatively large and the closing unit of FIG. 53B has substantially the same disadvantages as the aforementioned closing unit of FIG. 56.

SUMMARY OF THE INVENTION

The object of the present invention is to create a compact closing unit which is excellently suitable for injection molds of different length. This object is achieved by a closing unit in accordance with claim 1.

The closing unit of the invention comprises, in known manner, a fixed injection plate with injection opening and a movable closure plate. A displacement device for the movable closure plate produces the opening and closing movements and makes it possible to position the movable closure plate relative to the fixed injection plate. Several hydraulic force cylinders are arranged on the stationary injection plate in order to produce the closing force. For each force cylinder, the closure plate has a connecting rod for transmitting a closing force from the piston of the force cylinder to the movable closure plate. The closing unit of the invention differs from the known closing unit which has the features indicated above primarily by locking bushings on the injection plate. These locking bushings are mounted rotatably around their axis and in each case mechanically connected with the piston of a force cylinder, the connecting rods passing axially through the hydraulic force cylinders and the locking bushings. The locking bushings can be turned by one or more actuators into a first and a second angular position. First locking means along a rod section A on the connecting rods and second complementary locking means in the locking bushing permit in the first angular position axial passage of the connecting rods through the hydraulic force cylinders and the locking bushings. In the second angular position, on the other hand, the first locking means in the rod section A on the connecting rods cooperate with the second locking means of the locking bushings for the transmitting of the required closing force.

The closing unit of the invention permits an extremely compact, space-saving construction. The required stroke of the force cylinders and thus their outside dimensions are minimum. As compared with known closing units, the closing unit of the invention is characterized by a very small structural length. The structural length of the closing unit need in fact not be substantially larger than the greatest possible distance between the injection plate and the closure plate. All hydraulic connections can be arranged on the fixed injection plate. Flexible hydraulic connections are, accordingly, not required. Nevertheless, the closing unit is excellently suited for receiving injection molds of different length. The range of length of the injection molds is limited here solely and exclusively by the length of the rod section A having the first locking means.

The first locking means advantageously comprise an outer toothing on the connecting rod, and the second locking means comprise an inner toothing in the locking bushing. Inner toothing and outer toothing are in this connection divided by longitudinal grooves into at least two rows of teeth. In the first angular position, the rows of teeth of the outer toothing can be passed through axially by longitudinal grooves of the inner toothing and the rows of teeth of the inner toothing can be passed through axially by the longitudinal grooves of the other toothing, so that an axial pushing through of the connecting rods through the hydraulic force cylinders and the locking bushings can (sic). In the second angular position, the teeth of the inner toothing, on the other hand, can engage behind the teeth of the outer toothing for the transmission of a pulling force.

The teeth of the inner and outer toothings can be arranged annularly or helically. In order to assure a dependable engagement of the inner toothing in the outer toothing, a substantially axial flank clearance should be developed between inner and outer toothings. A large flank clearance, however, has substantial disadvantages. For example, the operating stroke of the force cylinders is increased thereby and thus the consumption of energy by the closing unit. The flows through the force cylinders are considerably greater so that the hydraulic system of the closing unit must also be made larger. Furthermore, the locking bushings are relatively strongly accelerated upon overcoming a large flank clearance, so that the teeth of the inner toothing strike with great force against the teeth of the outer toothing.

Within the scope of this invention, however, there is proposed an extremely simple and inexpensive solution which completely eliminates the disadvantages of a large axial flank clearance. This solution consists essentially therein that the piston of the hydraulic force cylinder is coupled by a screw thread to the locking bushing and that the piston is secured against turning. Upon turning the locking bushing by an angle γ from the first angular position into the second angular position, the locking bushing accordingly experiences an advance X relative to the fixed piston. The pitch of the screw thread is then advantageously so designed that by turning the locking bushing from the first angular position into the second angular position, the existing axial flank clearance S between inner and outer toothings is distributed unilaterally in such a manner that no substantial flank clearance is present any more between the tooth flanks which are to transmit the force. If, in this connection, the actuator for the turning of the locking bushings is so designed that it can place the locking bushings into a second angular position both by counterclockwise rotation and by clockwise rotation, then the flank clearance S between inner and outer toothings is distributed, depending on the direction of rotation, on the one hand to the left side, and on the other hand to the right side. The toothings are accordingly automatically without clearance for the transmission of the closing force in the first direction of rotation and automatically free of clearance for the transmission of an opening force to the closure plate in the second direction of rotation.

The injection plate advantageously has rotatably mounted slide shoes as radial guide for the connecting rods. The longitudinal grooves in the outer toothing form guide surfaces for these slide shoes, which are extended beyond the rod section A.

One very advantageous embodiment of the actuator as well as a very advantageous hydraulic control of the closing unit will be described inter alia in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments, as well as various features and advantages, of the invention will be described in detail with reference to the figures of the accompanying drawings, in which:

FIG. 1 is a view of a closing unit in accordance with the invention;

FIG. 2 is a longitudinal section through the closing unit of FIG. 1;

FIGS. 3 and 4 are a cross section through a connecting rod, and a locking bushing of the closing unit of FIG. 1;

FIG. 5 is a cross section through the connecting rod;

FIG. 6 is a cross section through the locking bushing;

FIG. 7 is a section along the section line A—A of FIG. 3;

FIG. 8 is a section along the section line B—B of FIG. 4;

FIG. 9 is an enlargement from the longitudinal section of FIG. 2;

FIG. 10 is a section through an actuator for two locking bushings;

FIGS. 11 to 14 are cross sections through different embodiments of the connecting rod;

FIGS. 15 to 20 are 180° developments of the toothings on the connecting rod and the locking bushing; and

FIG. 21 is a diagram of the hydraulic control of the closing unit.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

The general construction of a closing unit 10 in accordance with the invention will be explained with reference to FIGS. 1 and 2. An injection plate 14 having a central injection opening 16 is rigidly mounted on a base 12. The base 12 forms a guide bed 18 for a movable closure plate 20. The complementary halves of an injection mold (not shown) are clamped onto injection plate 14 and closure plate 20. The movable closure plate 20 is displaceable via an actuating device which comprises, for instance, two laterally arranged displacement cylinders 25. It is guided in this connection in the base 12. The displacement cylinders 25 accordingly open and close the complementary halves of the injection mold by displacement of the closure plate 20 relative to the injection plate 14. The housing of the displacement cylinders 25 is fastened in each case on the stationary injection plate 14 so that both displacement cylinders 25 have a rigid hydraulic connection on the fixed end plate 14.

Four connecting rods 22 extend from the movable closure plate 20 through the stationary injection plate 14. The connecting rods 22 are rigidly fastened to the closure plate 20. On the stationary injection plate 14, there is associated with each connecting rod 22 a force cylinder 26 the housing of which is rigidly connected to the injection plate 14.

In FIG. 2 and in FIG. 9 it can be seen that each of these force cylinders 26 comprises an annular piston 28. Each of these pistons 28 is rotatably connected to a locking bushing 34. The connecting rods 22 pass axially in this connection through the hydraulic force cylinders 26 and the locking bushings 34. First locking means 36 are provided on the connecting rods 22 in each case along a rod section A. Each of the locking bushings 34 has within it second locking means 38 complementary to the first locking means 36. These first and second locking means 36 and 38 are so developed that in a first angular position of the locking bushing 34 the connecting rod 22 can pass axially through the locking bushing 34, in which connection the second locking means 38 can, however, be brought, by turning the locking bushing, into a second angular position in which they cooperate within the rod section A with the first locking means 36 in order to transmit the required closing force.

In each force cylinder a first pressure chamber 30 is axially limited on the one hand by the injection plate 14 and on the other hand by the piston 28. If, after locking the locking bushings 34 in the rod section A, the pressure chamber is placed under pressure, then the piston 28 exerts an axial pulling force on the connecting rods 22 via the locking bushings 34 which are locked on the connecting rods 22, in which connection the force of reaction is taken up by the stationary injection plate 14. This first pressure chamber 30 accordingly produces the closing force necessary to lock the injection mold. Within a second pressure chamber 31, the piston 28 forms a substantially smaller pressure surface as shoulder surface. If this second pressure chamber 31 is placed under pressure and the first pressure chamber relieved, then the piston 28 exerts an axial pressing force on the connecting rods 22 in the direction opposite the closing force described above. This pressing force serves to open the mold after the casting.

For the displacement of the movable closure plate 20 on the guide bed 18 by means of the displacement cylinders 25, the locking bushing 34 is in the first angular position. In this angular position, the connecting rods 22 slide axially through the locking bushings 34 upon displacement of the closure plate 20. When the closure plate 20 has reached its intended position on the guide bed 18, the connecting rods 22 are locked in the locking bushings 34 of the movable closure plate 20 by turning the locking bushings 34 from the first angular position into the second angular position. The four force cylinders 26 can now transmit the required closing force via the connecting rods 22 to the closure plate 20, i.e. pull the closure plate 20 in the direction towards the injection plate.

One advantageous embodiment of the locking means will be described in further detail with reference to FIGS. 2 to 8. The locking means in the rod section A on the connecting rod 22 advantageously comprise (see FIGS. 2 and 5) an outer toothing 40 which is divided by longitudinal grooves 42 into, for instance, three axial rows of teeth 40 ₁, 40 ₂, 40 ₃. In these rows of teeth 40 ₁, 40 ₂, 40 ₃, the teeth of the outer toothing are arranged in each case parallel to and at the same distance from each other. The locking bushings 34 (see FIGS. 9 and 6) comprise a complementary inner toothing, which is also divided by longitudinal grooves 44 ₁, 44 ₂, 44 ₃, into three axial rows of teeth 46 ₁, 46 ₂, 46 ₃. The longitudinal grooves 42 _(i) in the outer toothing of the connecting rod 22 are somewhat wider than the teeth 46 _(i) of the locking bushing 34, and the longitudinal grooves 44 _(i) in the inner toothing of the locking bushing 34 are somewhat wider than the teeth 40 _(i) of the connecting rod 22.

In a first angular position of the connecting rod, shown in FIG. 3, the teeth 46 _(i) of the outer toothing of the rack (sic) 22 lie in the longitudinal grooves 44 _(i) of the locking bushing 34. In this angular position the connecting rod 22 can be pushed through the locking bushing 34, the teeth 40 _(i) of the outer toothing being guided by the longitudinal grooves 44 _(i) of the inner toothing, and the teeth 46 _(i) of the inner toothing being guided by the longitudinal grooves 42 _(i) of the outer toothing. FIG. 7 shows, in a cross section along the section line A—A of FIG. 3, the teeth of the inner toothing in the longitudinal grooves of the outer toothing.

In a second angular position—see FIG. 4—after the turning of the locking bushing 34 by an angle γ=180°/n (n=number of longitudinal grooves or of rows of teeth), the teeth 46 _(i) of the locking bushing 34 are located axially between the teeth 40 _(i) of the connecting rod 22. In this second angular position, therefore, the rows of teeth of the inner toothing engage into the rows of teeth of the outer toothing for the transmission of the necessary closing force.

FIG. 8 shows a section along the section line B—B of FIG. 4. It can be seen that the teeth of the outer and inner toothings have a trapezoidal cross section. The toothings can be developed helically, i.e. the teeth arranged along a helical line, and the toothings accordingly form a thread having a pitch P. The toothings can, however, also be annular, i.e. the teeth can form parallel rings which are arranged in each case at a distance P apart (also called pitch P).

In order that the inner toothing can engage into the outer toothing upon the turning of the locking bushing 34, the teeth 46 _(i) of the inner toothing must, of course, be axially between the teeth 40 _(i) of the outer toothing in the first angular position of the locking bushing 34. In order that small errors in position of the movable closure plate do not prevent engagement of the inner toothing into the outer toothing, a relatively large axial flank clearance is desired between the inner toothing and the outer toothing.

From FIG. 8 it can be seen that P=2D+S

in which:

P=pitch;

D=average tooth width;

S=axial flank clearance.

In practice, it has proven to be advantageous for S to be equal to 0.5D, and therefore P to be equal to 2.5D.

The four locking bushings 34 transmit extremely high closing forces via the connecting rods 22 to the movable closure plate 20. In addition, the frequency in actual practice of the closings and openings is very high. The locking bushings 34 and connecting rods 22 are accordingly subjected to extremely high loads and permanent deformation can occur which impair the operation of the locking device. In order to reduce the negative effects of such permanent deformations on the locking function, the following measures can advantageously be taken:

a) The locking bushings 34 are so fastened to the piston 28 that they are under tensile stress upon transmission of the closing force to the connecting rods 22. In this way, the result is obtained that both the connecting rods 22 and the locking bushings 34 are lengthened in the same direction, i.e. are uniformly deformed.

b) The cross sections of the locking bushings 34 and of the connecting rods 22 should be so developed that they are subjected to approximately the same tensile stresses upon transmission of the closing force.

c) With the same tooth geometry, the base of the teeth of the outer toothing should be approximately equal to the base of the teeth of the inner toothing, so that the stress maxima at these critical places are approximately the same. This means, for instance, that the arc length (in degrees) of the teeth of the outer toothing is greater than the arc length of the teeth of the inner toothing.

d) The teeth of the outer toothing should be of a greater hardness than the teeth of the inner toothing. In this connection, the flank surface of the teeth of the outer toothing should also be larger than the flank surface of the teeth of the inner toothing so that an imprint of the teeth of the outer toothing on the softer teeth of the inner toothing is avoided.

e) The elastic limit of the connecting rods 22 should be about 20% greater than the elastic limit of the locking bushings 34. In this way, in combination with measures b) and c), the result is obtained that plastic deformations upon overloading occur, particularly on the locking bushings 34 and less so on the connecting rods 22. Plastic deformations on the connecting rods 22 are far more disturbing, since they namely destroy the axial homogeneity of the outer toothing in the rod section A, which can lead to inaccuracies in the positioning of the closure plate 20 if molds of different size are used. Furthermore, the replacement of the connecting rods 22 is far more expensive than the replacement of the locking bushings 34.

It should be pointed that measures a), b) and c) of the above enumeration have advantageous effects on the distribution of the tensile force also in the normal case of elastic deformation. The elastic deformation of the locking bushings and the elastic deformation of the connecting rods are caused by these measures to take place in the same direction and be of the same order of magnitude, so that the tensile force to be transmitted is distributed over all interengaged teeth of the outer and inner toothings.

In the description of FIG. 8, it was pointed out that substantial flank clearance has the advantage that small inaccuracies in the positioning of the closure plate 20 by the displacement cylinders 25 do not prevent engagement of the inner toothing of the locking bushings 34 into the outer toothing of the connecting rods 22. However, a substantial axial flank clearance also has essential disadvantages. First of all, the stroke of the piston 28 increases with the flank clearance S, as a result of which the consumption of oil and energy by the force cylinders becomes greater. Secondly, the locking bushings 34 are initially imparted a high acceleration when acted on with pressure by the force cylinders 28, so that the teeth of the inner toothing strike strongly against the teeth of the outer toothing. For this reason, it is advantageous to provide a device which reduces or eliminates the flank clearance in the direction of the transmission of force.

One extremely advantageous development of this device is described with reference to FIG. 9. The locking bushing 34 is coupled turnably to the annular piston 28 via a thread 50 (hereinafter called the screw thread 50); for instance, it is screwed by means of the screw thread 50 into the free end of the annular piston 28. The latter is secured against turning, for instance by a spline 52. If, accordingly, the locking bushing 34 is turned by an angle γ it will experience an advance relative to the annular piston 28 of:

X=(γ/360°)P′

in which P′ is the pitch of the screw thread 50.

The turning of the locking bushing 34 is effected via a turning device 54 which is arranged in the extension of the force cylinder 26. This turning device 54 comprises a housing 56 which is, for instance, flanged onto the housing of the force cylinder 26. A toothed bushing 58 is arranged, turnable in a ball bearing 60, within the housing 56. The toothed bushing 58 is placed on the free end of the locking bushing 34 and so attached to such end via a tooth or spline-shaft connection that a moment of rotation is transmitted in form-locked manner, but at the same time an axial displacement of the locking bushing 34 in the toothed bushing 58 is possible. The angular position of the toothed bushing 58, and thus the angular position of the locking bushing 34, can be adjusted via an actuator 70 (see also FIG. 10) which engages into the outer toothing 64 of the toothed bushing 58. It should be pointed out that a pure moment of rotation is transmitted to the locking bushing 34. All radial forces which act on the toothed bushing 58 are transmitted directly by the ball bearing 60 to the housing 56. In this way assurance is had that the screw thread 50 is not stressed further by setting forces.

FIG. 10 shows an advantageous development of an actuator 70 for two toothed bushings 58′ and 58″ each. This actuator 70 comprises a rack 72, the toothing 74′ of which can engage into the toothed bushing 58′ and the toothing 74″ of which can engage into the toothed bushing 58″. The rack 72 is arranged in a housing tube 73. In each end of the rack 72 there is a cylindrical bore 76′, 76″. Pistons 78′, 78″ are introduced, sealed-off, into the respective cylinder bores 76′, 76″. These pistons are advantageously developed as plunger pistons. The pistons 78′, 78″ are flanged axially onto the two ends of the housing tube 73. The rack 72 is displaceable back and forth axially in the housing tube 73 between the two pistons 78′, 78″. In FIG. 10, the rack is shown resting against the left piston 78′; arrow 80 indicates the possible stroke of the rack 72 in the direction of the right piston 78″.

Both pistons 78′, 78″, which also have a guide function for the rack 72, have an axial connecting channel 82′, 82″ for a pressure fluid. Via these connecting channels 82′, 82″, the cylinder bores 76′, 76″ can be acted on optionally by the pressure fluid behind the pistons 78′, 78″ so that two oppositely acting pressure cylinders are developed for the displacement of the rack 72. It should be noted that these two pressure cylinders are arranged directly above the toothed bushings 58′, 58″. In this way, the structural length of the actuator is reduced to a minimum. It should also be noted that the actuators are so designed that the connecting rods 22 are turnable in each case from the first angular position to the right and to the left by an angle γ.

From FIG. 9 it can be seen that both on the injection plate 14 and on the housing 56 of the rotary device 54 for the locking bushings 34, a guide device 90 for the connecting rod 22 is provided. Each of these guide devices 90 comprises, for instance, three slide shoes 96. As can be noted from FIG. 2, the three longitudinal grooves 42 on the connecting rods 22 are developed as guide surfaces for these slide shoes 96 and are extended over the rod section A. The connecting rods 22 are centered in the locking bushings by these two guide devices 90.

FIGS. 11 to 14 show various embodiments of a connecting rod 22 as well as various arrangements of the slide shoes 96 and embodiments of the guide surfaces for the slide shoes 96. In accordance with the embodiment shown in FIG. 11, which is suitable for closing units of relatively small closing force, the connecting rod 22 comprises two longitudinal grooves 42 ₁, 42 ₂ which divide the outer toothing into two rows of teeth 40 ₁, 40 ₂. The slide shoes 96 ₁, 96 ₂ are guided in guide channels in the longitudinal grooves 42 ₁, 42 ₂. Corresponding to the embodiment shown in FIG. 12, the connecting rod comprises three longitudinal grooves 42 ₁, 42 ₂, 42 ₃ which divide the outer toothing into three rows of teeth 40 ₁, 40 ₂, 40 ₃. The guide surfaces for the slide shoes 96 ₁, 96 ₂, 96 ₃ are developed as flat surfaces which are at an angle of 120° to each other. The embodiment in accordance with FIG. 13 differs from the embodiment of FIG. 12 in the manner that the connecting rod 22 has four guide surfaces 42 ₁, 42 ₂. 42 ₃, 42 ₄ which are at an angle of 90° to each other. In accordance with FIG. 14, the outer toothing is divided by six longitudinal grooves into six rows of teeth; however only every second longitudinal groove is developed as guide surface for a slide shoe 96 ₁, 96 ₂, 96 ₃. It is self-evident that larger closing units require more rows of teeth and slide shoes than smaller closing units do.

On the basis of FIGS. 15 to 20, the design of the pitch of the thread 50 for the taking up of the axial flank clearance S will be explained in further detail. These figures show in each case a 180° development of the outer and inner toothings of FIGS. 3 and 4. There can be noted two of the three rows of teeth of the outer toothing of the connecting rod 22 and one of the three rows of teeth of the inner toothing of the locking bushing 34. The teeth of the inner toothing are shown hatched. The following designations are used in the drawings:

P: pitch of the outer toothing on the connecting rod 22, or of the inner toothing on the locking bushing 34;

D: average tooth width;

S: axial flank clearance between inner toothing and outer toothing;

P′: pitch of the screw thread 50 between connecting rod 22 and piston 28.

FIGS. 15, 17 and 19 show the position of the inner toothing before and after a 60° rotation of the locking bushing 34 in counterclockwise direction. Before the 60° rotation, the teeth of the inner toothing lie in a first angular position in the longitudinal grooves between the rows of teeth of the outer toothing. After this 60° rotation in counterclockwise direction, the teeth of the inner toothing lie in the second angular position with their left flanks against the teeth of the outer toothing and can transmit a force to the left without play from the locking bushing to the connecting rod. FIGS. 16, 18, and 20 show the position of the inner toothing before and after a rotation of the locking bushing by an angle of 60° to the right. Before the 60° rotation, the teeth of the inner toothing lie in a first angular position in the longitudinal grooves between the rows of teeth of the outer toothing. After this 60° rotation in clockwise direction, the teeth of the inner toothing lie in a second angular position with their right flank against the teeth of the outer toothing and can without play transmit a force to the right from the locking bushing to the connecting rod. For the designing of the pitch of the thread 50 for the taking up of the axial flank clearance S, it is assumed that, in the starting position, before the turning of the locking bushing, the rows of teeth of the inner toothing are in each case angularly precisely in the center between the rows of teeth of the outer toothing, and that the axial flank clearance S between inner toothing and outer toothings is distributed equally on both sides.

In the general case, the pitch of the screw thread is so designed that by turning the locking bushing from the first angular position into the second angular position, the existing flank clearance S between inner and outer toothings is distributed unilaterally in such a manner that no essential flank clearance is present any longer between the tooth flanks which are to transmit force.

FIGS. 15 and 16 refer to the case of an annular toothing. The pitch of the screw thread 50 is so designed that, by turning the locking bushing from the first angular position into the second angular position, the advance of the locking bushing corresponds approximately to half of the flank clearance S between inner and outer toothings, i.e.:

P′/6=0.5S or P′=3S;

for the special case that S=0.5D, i.e. S=P/5, we have accordingly:

P′=0.6P.

FIGS. 17 and 18 refer to the case of a helical toothing which ascends in direction of rotation of the locking bushing in the direction of the force to be transmitted. If it is assumed that the pitch P′ of the screw thread also ascends in the direction of rotation of the locking bushing in the direction of the force to be transmitted, then the advance of the locking bushing must correspond approximately to half of the flank clearance S between inner and outer toothings plus one-sixth of the pitch P of the toothing, i.e.:

P′/6=0.5S+P/6 or P′=3S+P.

For the special case of S=P/5, i.e. S=0.5D, we have accordingly:

P′=1.6P.

FIGS. 19 and 20 refer to the case of a helical toothing which has a negative pitch in the direction of turning of the locking bushing in the direction of the force to be transmitted. Furthermore, in FIGS. 19 and 20, the toothing is developed with a double thread, i.e. S=0.5P−2D. If one proceeds from the basis that the pitch P′ of the screw thread has a positive pitch, then the advance X of the locking bushing must correspond approximately to half of the flank clearance S between inner and outer toothings minus one-sixth of the pitch P of the toothing, i.e.:

P′/6=0.5S−P/6 or P′=4S−P;

for the special case of S=P/10, i.e. D=P/5, we have:

P′=−0.7P.

The minus sign in this case means that the screw thread 50 must also have a negative pitch.

FIG. 21 shows a block diagram of the hydraulic control of the closing unit 10. A 4/3-way proportional valve 100 has its first work outlet A connected in each case via a 2/2-way switch valve 102 ₁, 102 ₂, etc. to the first pressure chamber 30 of each of the four force cylinders 28. The 4/3-way proportional valve 100, which is controlled by a controller 103, controls by its work outlet A, upon the closing process, the closing pressure in the first pressure chamber 30 of the four force cylinders 26 as a function of a predetermined closing force 104. The work outlet B of the 4/3-way proportional valve 100 is connected directly to the second pressure chamber 31 of each of the four force cylinders 26.

The closure plate 20 is provided with a position sensor 105 which is connected to an axis control 106. A position sensor 108 is associated with the piston 28 of each force cylinder 26. The output signals S1, S2, S3, S4 of these position sensors 108 are also input values of the axis control 106. The reference numeral 110 indicates an input unit for the length “1” of the injection mold, i.e. the axial distance between closure plate 20 and injection plate 14. This length “1” is set by the axis control 106, via its output 112, the latter controlling the two displacement cylinders 25.

Before the turning of the locking bushing 34 from the first angular position into the second angular position, the teeth of the inner toothing should be positioned precisely axially between the teeth of the outer toothing of the locking bushing 34 in order to permit the proper engagement of the inner toothing into the outer toothing upon the turning of the locking bushing 34 into the second angular position. In order to make this axial positioning of the toothings possible independently of the length “1” set, the position of rest of the piston 28 as a function of the length “1” set is established hydraulically within a range [−0.5P;+0.5P] around a predetermined reference position. In other words, the locking bushing 34 is displaced axially, relative to a reference point, by an amount y, in which connection −0.5P<y<+0.5P. All actual positions of the four pistons 28 are compared for this purpose in the axis control 106 with the calculated desired position. The axis control 106, via the outputs V11, V12, V13, V14 controls the 2/2-way switch valves. If the measured actual position of a piston 28 corresponds to the predetermined desired position, the corresponding 2/2-way valve 102 is closed. This control permits, at little expense, a continuous adjustment of the length “1”, regardless of the pitch of the inner and outer toothings. It is pointed out that only one proportional valve is used for the control described above. 

What is claimed is:
 1. A closing unit of an injection molding machine comprising a fixed injection plate with injection opening and a movable closure plate which form clamping plates for an injection mold, a displacement device for the movable closure plate for the positioning of the movable closure plate relative to the fixed injection plate, a plurality of hydraulic force cylinders on the fixed injection plate to produce a closing force, the closure plate having one connecting rod per cylinder for transmitting the closing force from a piston of the force cylinder to the movable closure plate, wherein locking bushings are provided on the injection plate which are turnable around their axes and which are in each case connected with the piston of a respective one of the hydraulic force cylinders, the connecting rods passing axially through the hydraulic force cylinders and the locking bushings, an actuator for turning the locking bushings into a first and a second angular position, first locking means along a rod section A on the connecting rods and second locking means in the locking bushing, said first and second locking means registering with each other in such a manner that in the first angular position, they permit an axial passage of the connecting rods through the hydraulic force cylinders and the locking bushings, and that in the second angular position the first locking means in the rod section A on the connecting rods cooperates with the second locking means of the locking bushings for the transmission of the required closing force; and, the piston of the hydraulic force cylinder is connected via a screw thread to the locking bushing, the piston being secured against rotation, so that by the turning of the locking bushing around an angle γ from the first angular position into the second angular position, the connecting rod is advanced relative to the piston.
 2. A closing unit according to claim 1 wherein the first locking means comprise an outer toothing on the connecting rod and the second locking means comprise an inner toothing on the locking bushing, said inner toothing and said outer toothing being subdivided by longitudinal grooves into at least two rows of teeth in such a manner that, in the first angular position, the rows of teeth of the outer toothing can move axially through longitudinal grooves of the inner toothing and the rows of teeth of the inner toothing can move axially through the longitudinal grooves of the outer toothing and thus permit an axial pushing of the connecting rods through the hydraulic force cylinders and the locking bushings, and that in the second angular position the teeth of the inner toothing can engage behind the teeth of the outer toothing in order to transmit a tensile force.
 3. A closing unit according to claim 2, wherein an axial flank clearance S which corresponds approximately to half of the average width of a tooth is present between the outer toothing and the inner toothing.
 4. A closing unit according to claim 2, wherein the teeth of the inner toothing and the teeth of the outer toothing are arranged annularly.
 5. A closing unit according to claim 2, wherein the teeth of the inner toothing and the teeth of the outer toothing are arranged helically.
 6. A closing unit according to claim 2, wherein the teeth of the inner toothing and the teeth of the outer toothing have a trapezoidal cross section.
 7. A closing unit according to claim 6, wherein the actuator is designed for turning the locking bushings either way from the first angular position to cause the locking bushings to advance in a closing direction and in an opening direction.
 8. A closing unit according to claim 7, wherein the pitch of the screw thread is so designed that by turning the locking bushing out of the first angular position into the second angular position, an existing flank clearance S between the inner and outer toothings is distributed on one side in such a manner that no substantial flank clearance is present any longer between the two flanks which are to transmit the force.
 9. A closing unit according to claim 7, in which the teeth of the inner and outer toothings are arranged annularly, wherein the pitch of the screw thread is designed in such manner that by turning the locking bushing from the first angular position into the second angular position the advance of the locking bushing corresponds to approximately one half of the flank clearance S between inner and outer toothings.
 10. A closing unit according to claim 7, in which the teeth of the inner and outer toothings form a thread which in the direction of rotation has a positive pitch P in the direction of the force to be transmitted, wherein the pitch of the screw thread is so designed that by turning the locking bushing by an angle γ from the first angular position into the second angular position, the advance of the locking bushing corresponds approximately to half of the flank clearance S between inner and outer toothings plus (γ/360°)P.
 11. A closing unit according to claim 7, in which the teeth of the inner and outer toothings form a thread which in the direction of rotation has a positive pitch P in the direction of the force to be transmitted, wherein the pitch of the screw thread is so designed that by turning the locking bushing by an angle γ from the first angular position into the second angular, the advance of the locking bushing corresponds approximately to half of the flank clearance S between inner and outer toothings minus (γ/360°)P.
 12. A closing unit according to claim 10, in which the thread is developed as a double thread.
 13. A closing unit according to claim 2, wherein the locking bushing is so arranged on the piston of the force cylinder that it is under tensile stress upon transmission of the closure force.
 14. A closing unit according to claim 13, wherein the connecting rod has an elastic limit which is about 20% greater than that of the locking bushing, the connecting rod and the locking bushing being so designed that the maximal tensile stresses thereon upon transmission of the closing force are approximately equal.
 15. A closing unit according to claim 13, wherein the length of the base of a tooth of the inner toothing is equal to the length of the base of a tooth of the outer toothing.
 16. A closing unit according to claim 13, wherein the teeth of the inner toothing have a smaller flank surface than the teeth of the outer toothing, and the teeth of the outer toothing are of greater hardness than the teeth of the inner toothing.
 17. A closing unit according to claim 2, wherein slide shoes are arranged on the injection plate as radial guide for the connecting rods, the longitudinal grooves in the outer toothing of the connecting rods act as guide surfaces for said slide shoes and the guide surfaces extend over the rod section A with the outer toothing.
 18. A closing unit according to claim 15, further comprising a connecting rod lead-through in the injection plate extending in an axial direction and having: a. a ring segment with slide shoes on an entrance side; b. an annular force cylinder; c. a rotatably mounted locking bushing; d. a turning device for the turning of the locking bushing into the first and the second angular positions; and e. a ring segment with slide shoes or an outlet side.
 19. A closing unit according to claim 1, wherein the actuator in each case has a common rack for two locking bushings each.
 20. A closing unit according to claim 19, wherein the rack has a cylinder bore at each end, a fixed piston is introduced in sealed manner into each of the two cylinder bores so that the rack is displaceable axially back and forth between the two fixed pistons, and the two cylinder bores can be acted on behind the piston by a pressurized fluid so that two oppositely directed pressure cylinders are formed for the displacement of the rack.
 21. A closing unit according to claim 20, wherein each piston has an axial connecting channel for the pressurized fluid.
 22. A closing unit according to claim 20, further having a drive bushing with outer toothing which engage in form-locked manner the teeth of the rack, the drive bushing being rotatably mounted in a housing and having an opening for the axial introduction of the locking bushing, and by coupling means for the form-locked transmission of a moment of rotation from the drive bushing to the locking bushing introduced into the drive bushing, said coupling means permitting an axial displacement of the locking bushing in the drive bushing.
 23. A closing unit according to claim 22, wherein the coupling means are a tooth- or spline-shaft connection.
 24. A closing unit according to claim 1, wherein the force cylinders are developed as double-acting annular pressure cylinders with, in each case, a first pressure chamber for locking the injection mold and a second pressure chamber for opening the injection mold.
 25. A closing unit according to claim 24, further comprising a 4/3-way proportional valve having a first work outlet and a second work outlet, the first work outlet being connected via 2/2-way valves with the first pressure chambers of all force cylinders and the second work outlet being connected with the second pressure chambers of all force cylinders.
 26. A closing unit according to claim 2, further having a position sensor for sensing the actual position of the piston of each force cylinder, a 2/2-way valve for each force cylinder, and a control unit which closes the 2/2-way valve when the actual position of the piston corresponds to a predetermined desired position.
 27. A closing unit according to claim 2, further having a control unit for the axial positioning of the pistons of the force cylinders in a position of rest which is calculated in such a manner that upon the turning of the locking bushings from the first angular position into the second angular position, the teeth of the inner toothing lie axially between the teeth of the outer toothing.
 28. A closing unit according to claim 2, further having a position sensor for sensing the position of the closure plate, a position sensor for sensing the actual position of the piston of each force cylinder, a calculating unit for calculating a position of rest of the pistons as a function of the measured position of the closure plate, in such a manner that before the engagement of the inner toothing of the locking bushings into the outer toothing of the connecting rods, the teeth of the inner toothing lie axially between the teeth of the outer toothing, and a control unit with the measurement values of the position sensors of the pistons as input signals, for positioning the pistons into the calculated position of rest. 